SET Preamps

Not sure how you are defining "SET preamp," but as I would define it most and perhaps nearly all tube preamps are SETs.

Meaning that their output tubes (and other tubes that are in the signal path) are triodes, and the output stages do NOT consist of two triodes, or two triode sections, operated as push-pull pairs. Push-pull, btw, is NOT the same thing as differentially balanced, in case that is not clear.

The phrase "SET" is normally used to distinguish power amplifier output stages that consist of triodes that are not operated push-pull (and which usually provide very low power capability), from other output stage topologies that are operated push-pull and/or are based on tetrodes or pentodes.

Regards,
-- Al

Why is a simple single-ended triode able to give 21st-century state-of-the-art performance in a preamp circuit, while similar performance in the power amp seems to require ultralinear push-pull circuits or something even fancier?

Simply put, the power amp has to deliver vastly greater amounts of current and power to the speaker than the preamp has to deliver to the power amp. Which makes it a whole different ballgame.

Regards,
-- Al